"Johnny's's" is not correct. In fact, I think there probably isn't a correct way to use the Saxon genitive here. There are several ways to express the sentiment though:
- How is the pasta at Johnny's
- How is Johnny's restaurant's pasta?
- How is Johnny's pasta?
This last one is interesting. It can be interpreted two different ways. Either you are breaking out the restauranteur's name, presumably Johnny's is eponymous for the chef, so you are asking how is the chef's pasta. Or alternatively, we can use apposition here to imply the extra genitive relationship.
Me? I'd chose the first one. It is simplest.